Conventional consumer cameras (both film and digital) capture an image having a rectangular imaging area. For cameras using 35 mm film, the horizontal dimension of the imaging area is approximately 36 mm and the vertical dimension of the imaging area is approximately 24 mm.
Many photographers find it very difficult to hold a camera level when capturing a photograph. A photographer often pivots the camera slightly about the optical axis (whereby the optical axis is defined as an axis passing through the center of the image capture frame and the center of the camera lens). These small rotations are generally unintentional and may occur because conventional consumer cameras are lightweight, for example, the popular Single-Use cameras. Rotations about the optical axis give the appearance that the subjects in the photograph are off-centered, leaning, or tilted.
U.S. Pat. No. 6,011,585 issued Jan. 4, 2000 to Anderson, entitled Apparatus and Method for Rotating the Display Orientation of a Captured Image describes a method of determining image format and orientation from a sensor present in the camera at the time of image capture. Whereas, this sensor can enable one to determine the orientation or format of a captured image, it cannot lead to detecting a small amount of camera rotation. Instead, the sensor identifies the major image orientation (in increments of 90 degrees) by determining which side of an image corresponds with the “top” of the image. In addition, the Anderson method necessitates a sensor of this sort be present within a camera.
Lutton et al. (in “Contribution to the Determination of Vanishing Points Using Hough Transform,” IEEE Trans. Pattern Analysis and Machine Intelligence, Vol. 16, No. 4, pp. 430–438, Apr. 1994) attempts to detect the vertical direction of an image. The Lutton et al. article teaches one to select the direction that is orthogonal to the most directions in the scene. The implicit assumption is that the scene will contain many horizontal lines. However, this is not always the case. Additionally, the Lutton analysis is performed with a possibly large number of line segments, rather than vanishing points. The Lutton method requires a great deal of processing and may be computationally costly.
Some existing systems detect and correct a skewed image. These systems are primarily in the field of document imaging. Nevertheless, documents have a fairly well defined structure, as opposed to more unpredictable consumer type photographs. Another sharp contrast between document imaging correction and photographic imaging correction is that vanishing point detection is not used in the field of document imaging, nor is vanishing point detection required.
Pending U.S. application Ser. No. 09/281,574, filed Mar. 30, 1999 by Gallagher, entitled A Method for Modifying the Perspective of a Digital Image, shows a process for compensating from the perspective of image capture. However, this solution cannot lead to compensating for camera rotation about the optical axis. The perspective of image capture relates to tipping the camera in the up and down direction (i.e. about the x-axis), which is much different than rotating a camera about the optical axis. Therefore, pending U.S. application Ser. No. 09/281,574 is limited to describing a method of correction exclusively unique to the problem of camera rotation about the x-axis.
Consequently, a need exists for overcoming the above described drawbacks. More specifically, a need exists for determining slight image orientation changes.